1. Field of the Invention
The present invention relates to a self-supporting convex cover or radome for use as a protective cover for spacecraft hardware in the space environment, for example, for providing thermal stability and electrostatic charge dissipation. The radome is ideally also of low weight on earth, and can be made transparent to a wide range of radio frequencies if desired.
2. Description of the Related Art
Various types of equipment, such as communications equipment, require protection when placed in environments which are electrically charged and/or contain great thermal variations to avoid damage or distortion of the equipment. For example, the space environment subjects spacecraft exterior hardware, such as antennas, to great extremes in temperature and high fluxes of charged particles, or plasma. Great variations in temperature over short periods of time can cause mechanical distortions in the exterior hardware. Protecting communications hardware from extreme environments, such as space, presents a special problem because the materials and methods of construction normally used to protect spacecraft hardware interfere with radio signal transmissions. Also, the flat covers generally used in the prior art will focus reflected solar energy back on the spacecraft.
Although protection from the hostile environment is a primary concern, minimization of the cost of the cover and its total weight are competing concerns. The cover should also be self-supporting so that it does not collapse on the hardware it is intended to protect. When desired or necessary, it should also minimize interference and attenuation of radio signals due to the cover or its support structure. The radomes used in other environments typically are heavier than necessary in space, and the extra weight makes their use undesirable. Thus, there is a need for a radome that is self-supporting and lightweight, and yet relatively inexpensive, capable of providing electrical and thermal protection, and that can be made transparent to a wide range of radio frequencies.
Various types of radomes for use on earth, in the atmosphere, or in space are shown in the prior art which meet some of these criteria, but none meet all of them. For example, Boyd et al., U.S. Pat. No. 4,956,393, discloses the use of certain types of syntactic foams in radomes for use in the atmosphere. However, while a polycyanate resin could be used in the present invention, Boyd et al. does not disclose any construction other than a dome of constant thickness. Also, syntactic foams in general cause significant radio frequency losses due to their density.
Archer, U.S. Pat. No. 4,847,506, shows the use of tiles like those used on the space shuttle to protect the spacecraft hardware. While this fibrous silica material is somewhat like a foam, the use of such tile to make a radome makes the structure quite heavy. The use of this tile in Archer is not for protecting the hardware from the space environment, but from high energy laser radiation or nuclear radiation from "satellite killers."
Traut, U.S. Pat. No. 4,615,933, shows a process for manufacturing Teflon saturated glass fabric for microwave radomes. However, Traut is directed at aircraft applications and the material is heavier than desired for space applications.
Greene, U.S. Pat. No. 4,506,269, shows the use of a hard thermoplastic polycarbonate sandwich, again intended for use in the atmosphere, such as for use on a plane traveling at supersonic speeds. Thus, it is again far stronger and heavier than is necessary or desirable for space applications. Further, the ratio of core material to air must be adjusted to "tune" the frequency at which the radome is most transparent to microwaves.
Rogers et al., U.S. Pat. No. 4,479,131, shows a sun shield of a material somewhat similar to that used in the preferred embodiment of the present invention. However, to block the sun while achieving radio frequency transparency, Rogers et al. utilizes a capacitive grid of aluminum squares. Also, there is no self-supporting structure or convex shape in Rogers et al., as in the present invention. In fact, Rogers et al. teaches away from the present invention as it recommends the use of non-rigid materials which are not self-supporting.
McMillan et al., U.S. Pat. No. 2,956,281, shows the use of a "cellular" or foam dialectric material in radomes. An outer skin or, alternatively, ribs of a dialectric material may be used to provide structural strength. However, where ribs are used, the cellular material must be loaded with particles having a dialectric constant such that the overall dialectric constant of the radome remains uniform across the surface.
Black, U.S. Pat. No. 2,641,561, shows fiberglass records running through a uniformly thick radome of glass foam between fiberglass sheets. This radome is intended for use in airplanes, and is unacceptably bulky and heavy for space applications.
Japanese patent No. 59-16401 shows the use of foamed plastic balls to fill up an antenna dish to prevent the radome from touching the antenna itself. This is not only too bulky and heavy for space applications, but results in uneven support for the radome.
A radome is thus desired which does not focus sunlight onto the spacecraft and which is self-supporting, thin, light in weight, easy to manufacture, simple to attach and transparent to radio frequencies.